An active matrix liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. In the monolithic display device, it is the main stream, to employ the thin film transistors (TFTs). In the thin film transistor, an amorphous silicon film is formed on an insulating substrate such as a glass substrate or a quartz substrate to obtain an active layer. The development of a system-on-panel incorporating therein logic circuits such as a memory circuit and a clock generating circuit utilizing the TFTs has been advancing.
The high speed operation is required for such a driver circuit and the logic circuit, and therefore it is not suitable therefor that the amorphous silicon film is formed as the active layer on the quartz substrate or the glass substrate to obtain an element. For this reason, at present TFT in which a polycrystalline silicon film is used as the active layer is manufactured.
There are present some technologies in which after having deposited the amorphous silicon film on the quartz substrate or the glass substrate, the polycrystalline silicon film is obtained through crystallization. Of those, there is known a technique in which the catalytic metal element, with which the excellent electrical characteristics of the element are obtained when forming the element, and which promotes the crystallization of the amorphous silicon film, is added to the film to conduct the crystallization by a heat treatment. This technique will hereinbelow be described in more detail.
A semiconductor thin film having the amorphous structure containing as a main component a silicon is formed on an insulating substrate such as a quartz substrate or a glass substrate into a thickness on the order of 50 nm to 100 nm by LPCVD or the PECVD. Metal is added to the surface of the semiconductor thin film or into the semiconductor thin film having the above-mentioned amorphous structure to carry out the heat treatment therefor, thereby crystallizing the semiconductor thin film having the above-mentioned amorphous structure in the solid phase. The semiconductor thin film having the above-mentioned amorphous structure is crystallized in the solid phase so that a crystalline semiconductor thin film containing silicon as the main component is formed. Then, it is confirmed by the inventors of the present invention that the addition of the metal promotes the solid-phase crystallization, and it is therefore said that the metal acts as the catalyst during the solid-phase crystallization. In the present specification, the metal is referred to as the catalytic metal.
As for the phenomenon that the semiconductor thin film having the above-mentioned amorphous structure is crystallized by the heat treatment with the metal as a catalyst, a large number of reports have been made as the Metal Induced Lateral Crystallization (MILC). As the typical ones, there are the transition metal elements such as nickel (Ni), cobalt (Co), palladium (Pd), platinum (Pt), and copper (Cu). The presence of the catalytic metal becomes advantageous in the temperature and the time required for the semiconductor thin film having the above-mentioned amorphous structure to be crystallized in the solid phase as compared with the case where no catalytic metal is added. As a result, it has become clear that the Ni element shows excellent property as the catalytic metal. Hereinbelow, descriptions will be made on the assumption that the Ni element is employed as the catalytic metal.
The heat treatment required for the solid phase crystallization of the semiconductor thin film having the above-mentioned amorphous structure is performed at from 400° C. to 700° C. for several hours or more by the electric furnace, etc.
In the present specification, the semiconductor thin film having the amorphous structure containing silicon as the main component includes a SiGe thin film having the amorphous structure, in which the component ratio of Ge is less than 50%.